Cobalt-promoted fischer-tropsch catalysts

ABSTRACT

Iron-cobalt spinels which contain low levels of cobalt, in an iron/cobalt atomic ratio of 7:1 to 35:1, are converted to Fischer-Tropsch catalysts upon reduction and carbiding that exhibit high activity and selectivity to C2-C6 olefins and low CH4 production.

FIELD OF THE INVENTION

This invention relates to a Fischer-Tropsch process for producing lowmolecular weight olefins, particularly those in the C₂ -C₄ range, usingas a catalyst, an unsupported alkali or alkaline earth metal saltpromoted iron-cobalt single phase spinel, in which the atomic ratio ofFe:Co is 7:1 or above, and said spinel having a measured BET nitrogensurface area of up to about 5 m² /g.

DISCLOSURES IN THE ART

Fischer-Tropsch processes have long been known to produce gaseous andliquid hydrocarbons containing C₂ -C₄ olefins. Because of the importanceof C₂ -C₄ olefins, particularly as feedstocks for the chemical industry,modifications of the Fischer-Tropsch process are constantly beingpursued toward the goals of maximizing C₂ -C₄ olefin selectivity withthe particular objective of maintaining high catalyst activity andstability under the reaction conditions. The main thrust of the effortsin this area has been in the area of catalyst formulation.

Coprecipitated iron-based catalysts, including those containing cobalt,are known for producing C₂ -C₄ olefins. High levels of cobalt in aniron-cobalt alloy are known to produce enhanced selectivity to olefinicproducts, as described in Stud. Surf. Sci. Catal. 7, Pt/A, pp. 432(1981).

Other disclosures in the art directed to coprecipitated iron-cobaltcatalysts and/or alloys include: U.S. Pat. Nos. 2,850,515, 2,686,195,2,662,090, and 2,735,862; AICHE 1981 Summer Nat'l Meeting Preprint No.408, "The Synthesis of Light Hydrocrobons from CO and H₂ Mixtures overSelected Metal Catalysts" ACS 173rd Symposium, Fuel Division, NewOrleans, March 1977; J. Catalysis 1981, No. 72(1), pp. 37-50; Adv. Chem.Ser. 1981, 194, 573-88; Physics Reports (Section C of Physics Letters)12 No. 5 (1974) pp. 335-374; UK patent application No. 2050859A; J.Catalysis 72, 95-110 (1981); Gmelins Handbuch der Anorganische Chemie 8,Auflage (1959), pp. 59; Hydrocarbon Processing, May 1983, pp. 88-96; andChem. Ing. Tech. 49 (1977) No. 6, pp. 463-468.

There is further disclosed a method for producing high surface areametal oxides in the French article, "C. R. Acad. Sc. Paris", p. 268 (28May 1969) by P. Courte and B. Delmon. The article describes a processfor producing high surface area metal oxides by evaporating to drynessaqueous solutions of the corresponding glycolic acid, lactic acid, malicor tartaric acid metal salts. One oxide that was prepared by theirdescribed method was CoFe₂ O₄.

However, the above references do not describe or suggest the use ofsingle phase iron-cobalt spinels having an Fe:Co atomic ratio of 7:1 orabove or suggest their applicability in conducting or carrying outFischer-Tropsch processes for synthesizing C₂ -C₄ olefins.

What is particularly desired in fixed bed Fischer-Tropsch processes arenew catalysts for selectively producing high levels of C₂ -C₄ olefinsand low levels of methane under the desirable combined conditions ofhigh catalyst activity and stability.

SUMMARY OF THE INVENTION

It has been found that unsupported alkali or alkaline earth metal saltpromoted iron-cobalt single phase spinels containing low levels ofcobalt, i.e. iron:cobalt atomic ratios of 7:1-35:1 and higher providedesirable catalyst properties in fixed bed Fischer-Tropsch processes.The initial spinels are single phase and isostructural with Fe₃ O₄ asshown by X-ray diffractometry and possess measured BET nitrogen surfaceareas of up to 5 m² /g (square meters per gram).

The spinels are prepared in a high temperature solid state sinteringreaction in a temperature range of about 600° to 1100° C. betweenstoichiometric amounts of mixtures of the component metal oxides and/ormetals, in an inert or vacuum atmosphere. The spinels prepared in thismanner are then treated with promoter agents, alkali metal and alkalineearth metal salts, and particularly potassium carbonate. The resultingcombined iron and cobalt/potassium atomic ratio is desirably in therange of about 20:1 to 200:1. The promoted catalyst is then reduced in ahydrogen containing gas and carbided before use in the Fisher-Tropschprocess.

In accordance with this invention there is provided, a hydrocarbonsynthesis catalyst composition comprising an unsupported, Group IA orIIA metal salt promoted iron-cobalt single phase spinel, said spinelhaving the initial empirical formula:

    Fe.sub.x Co.sub.y O.sub.4

wherein x and y are integer or decimal values, other than zero, with theproviso that the sum of x+y is 3 and the ratio of x/y is 7:1 or above,said spinel exhibiting a powder X-ray diffraction pattern substantiallyisostructural with Fe₃ O₄ and said spinel having an initial BET surfacearea of up to about 5 m² /g.

Preferred embodiments of the composition include the substantiallyreduced and carbided form of the spinel, which is an activeFisher-Tropsch catalyst in fixed bed process for producing low molecularweight olefins.

Furthermore, there is provided a process for producing the subjectspinel portion of the composition comprising the step of heating amixture of cobalt and iron, as their oxides, free metals, or mixturesthereof, to produce the empirical composition: Fe_(x) Co_(y) O₄, where xand y are integers or decimal values, other than zero, and where the sumof x+y is 3, and the ratio of x/y is about 7:1, or above, for a timesufficient to produce said single phase spinel being isostructural withFe₃ O₄, and having a surface area of up to about 5 m² /g.

There is further provided a process for synthesizing a hydrocarbonmixture containing C₂ -C₆ olefins comprising the step of contacting acatalyst composition, comprised of an unsupported Group IA or IIA metalsalt promoted iron cobalt spinel, said spinel initially exhibiting asingle spinel phase, being isostructural with Fe₃ O₄, as determined byX-ray diffractometry, and possessing an initial BET nitrogen surfacearea of up to about 5 m² /g, and an iron-cobalt atomic ratio of 7:1 orabove, with a mixture of CO and hydrogen under process conditions ofpressure, space velocity and elevated temperature for a time sufficientto produce said C₂ -C₆ olefins.

DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS

The subject iron-cobalt spinels are new compositions of matter which areisostructural with Fe₃ O₄, as determined by x-ray diffractometry usingcopper K alpha radiation and exhibit a single spinel phase. By the term"spinel" is meant a crystal structure whose general stoichiometrycorresponds to AB₂ O₄, where A and B can be the same or differentcations. Included within this definition is the commonly found spinelMgAl₂ O₄. A and B can have the following cationic charge combinations:A=+2, B=+3, A=+4, B=+2, or A=+6, B=+1. Spinels are arranged of anapproximately cubic close-packed arrangement of oxygen atoms with 1/8thof the available tetrahedral interstices and 2/3 of the octahedralinterstices filled, and can exhibit hundreds of different phases.Further description of the spinel structure can be found in "StructuralInorganic Chemistry" by A. F. Wells, Third Edition, Oxford Press, andthe article "Crystal Chemistry and Some Magnetic Properties of MixedMetal Oxides With the Spinel Structure" by G. Blasse, Phillips ResearchReview Supplement, Volume 3, pp 1-30 (1964). By the term "isostructural"is meant crystallizing in the same general structure type in that thearrangement of the atoms remains very similar with only minor changes inunit cell constants, bond energies and angles. By the term "single phasespinel", as used herein, is meant one structural and compositionalformula, corresponding to a single spinel material into which all of themetal components are incorporated, and exhibiting one characteristicX-ray diffraction pattern.

The subject iron-cobalt spinel possesses a BET surface area up to about5 m² /g, as determined by the well-known nitrogen gas BET surface areameasurement technique as described in the reference JACS 60, p. 309(1938) by S. Brunauer, P. H. Emmett, and E. Teller. Generally, thespinel has a surface area of about 0.1 to 1 m² /g. This range of surfacearea generally corresponds to a particle size range of about 1 to 10microns.

The iron to cobalt atomic ratio of the metals in the spinel is about 7:1or above and is preferably in the range of about 7:1 to 35:1.

The spinel can be represented by the formula: Fe_(x) Co_(y) O₄, whereinx and y are decimal or integer values, other than zero, and wherein thesum of x plus y is 3, and the ratio of x to y is 7:1 or above andpreferably being about 7:1 to 35:1. Particularly preferred is where theiron to cobalt atomic ratio is about 19 to 20:1.

Representative examples of the various spinels corresponding to theformula are Fe₂.85 Co₀.15 O₄,Fe₂.625 Co₀.375 O₄, Fe₂.97 Co₀.03 O₄ andFe₂.25 Co₀.75 O₄.

Physical properties in general of these subject spinels are similar tothose of magnetite, Fe₃ O₄, and include: melting point of above 1400°C., and color of brownish to blackish.

The iron-cobalt spinels are used in unsupported form in H₂ /COhydrocarbon synthesis.

A promoter agent is also used in the composition and is used toparticularly promote olefin formation in the process. Representativeexamples of classes of suitable promoter agents include alkali metal andalklaine earth metal salts including carbonates, bicarbonates, organicacid salts, inorganic acid salts, i.e. acetates, nitrates, halides,sulfates, and hydroxide salts of Group IA and IIA metals includinglithium, sodium, potassium, cesium, rubidium, barium, strontium,magnesium, and the like. Preferably, the promoter agent is deposited orimpregnated substantially on the surface of said spinel composition.

Representative examples of specific promoter agents are potassiumcarbonate, potassium sulfate, potassium bicarbonate, cesium chloride,rubidium nitrate, lithium acetate, potassium hydroxide, and the like.Preferred are the Group IA compounds and a particularly preferredpromoter agent is potassium carbonate. The promoter, if used, isgenerally present in about a 0.1 to 10 gram-atom % as the metal ion ofthe total combined metal gram-atoms present. A preferred level ofpromoter agent is in the range of 1 to 2 gram-atom % of the totalcombined metal graom-atoms present. In the empirical formulas usedherein, the amount of the promoter agent, e.g., potassium, is expressedin terms of gram atom percent based on the total gram-atoms of metalsused. Thus, "1 gram-atom of potassium" signifies the presence of 1gram-atom of potassium per 100 total gram atoms of combined gram atomsof Fe and Co. Thus, the symbol "1% K" as used herein indicates 1gram-atom percent potassium based on each 100 gram atoms of the totalcombined gram atoms of iron and cobalt present.

A particularly preferred spinel composition of the subject invention isFe₂.85 Co₀.15 O₄ /1% K (potassium taken as the carbonate).

The catalyst spinel in the subject process may also be used inconjunction and admixture with a diluent material; one which aids inheat transfer and removal from the catalyst bed. Suitable materialsinclude powdered quartz, silicon carbide, powdered borosilicate glass,SiO₂, pourous silica, kieselguhr, zeolites, talc, clays, Group II to VIImetal oxides and rare earth oxides including TiO₂, SiO₂, Al₂ O₃, MgO,La₂ O₃, CeO₂, Cr₂ O₃, MnO₂, and the like. Preferred is powdered quartz.

The diluent, if used, is generally used in a 1:4 to 9:1 diluent/spinelcatalyst composition weight ratio. Preferred is a 1:1 weight ratio.

The utility of these spinels is their ability upon subsequent reductionand carbiding to form active catalysts in a fixed bed Fisher-Tropschprocess for making C₂ -C₄ olefins from CO/hydrogen.

The reduced and carbided forms of the above-described spinel are alsosubjects of this invention.

The subject spinel is prepared by a solid state high temperaturereaction between (1) the component oxides, i.e. Fe₃ O₄ and Co₃ O₄, or(2) a mixture of iron metal, cobalt oxide and iron oxide, i.e. Fe metal,Co₃ O₄ and Fe₂ O₃, or (3) a mixture of cobalt metal, iron oxides andcobalt oxide, i.e. Co, Fe₃ O₄, Fe₂ O₃ and Co₃ O₄ or (4) a mixture ofiron and cobalt metals, iron oxide and cobalt oxide, i.e. Fe, Co, Fe₂ O₃and Co₃ O₄, in the correct stoichiometric metals and oxygen ratio toresult in the empirical formula for the composition as given above.Preferred is indicated reaction (1) between iron oxide and cobalt oxide.The reaction is conducted at temperatures in the range of about 600° to1100° C. and preferably from about 800° to 900° C., in an inert gas,oxygen-free atmosphere, or vacuum environment. Examples of useful inertgases are helium, nitrogen, argon, and the like. The solid state hightemperature reaction "sintering" should be performed on thoroughly mixedsamples of the metal oxides and/or metal and metal oxide mixtures. Amethod of forming the mixture is by intimate grinding and shaking. Thesintering reaction should be conducted until a powder X-ray diffractionpattern indicates a single spinel phase is formed, being isostructuralwith Fe₃ O₄, which generally requires about an 8 to 24 hour period andpreferably about a 12 to 18 hour period. Generally, at the end of eachreaction period the material is thoroughly ground and mixed and thenresubjected to the high temperature conditions for an additional 1 to 5cycles or until powder x-ray diffraction reveals the presence of asingle spinel phase.

Prior to the hydrocarbon synthesis run, the iron-cobalt spinel isreduced in a reducing atmosphere at elevated temperature, generally in atemperature range of about 200° to 500° C. and preferably 350° to 450°C. The reduction can be carried out with various reducing gasesincluding hydrogen, CO, and mixtures thereof, and the like. Preferably,hydrogen gas, either by itself or in an inert carrier medium such ashelium, neon, argon, or nitrogen, is preferably used. The pressure ofthe reducing gas in this procedure may be in the range of 1.5 to 1000psig and preferably in the range of 15 to 150 psig. The reducing gasfeed rate may be in the range of 1-10,000 V/V/hr and preferably in therange of 10-1000 V/V/hr. The reduction is carried out until theresulting Fe-Co alloy is substantially reduced and exhibits a powderX-ray diffraction pattern isostructural with alpha iron. This reductionusually requires about 2-20 hours.

The resulting reduced spinel generally has a BET surface area of up to 3m² /g and is useful in forming a carbided iron-cobalt catalyst useful inthe subject Fischer-Tropsch process for making C₂ to C₆ olefins asdescribed herein.

The iron-cobalt catalyst which is believed to be the primary activecatalyst in the process can be produced by carbiding the reducediron-cobalt spinel, described hereinabove, generally having an X-raydiffraction pattern isostructural with chi Fe₅ C₂ (Hagg carbide), byheating at elevated temperature in a suitable carbiding atmosphere,containing CO, H₂ /CO, and mixtures thereof. The spinel can also bereduced and carbided, concurrently, by contact with a CO/H₂ atmosphereunder the hydrocarbon synthesis conditions described below.

Also a subject of the instant invention is a Fischer-Tropsch fixed bedprocess for producing C₂ -C₆ olefins by utilizing the reduced andcarbided iron-cobalt spinel, described hereinabove.

Although a fixed bed Fischer-Tropsch process is one desired mode forutilizing the claimed catalysts described herein, a slurry type processwherein the catalyst is suspended in a liquid hydrocarbon can also beutilized, as described in copending application, Ser. No. 561,192, filedDec. 14, 1983 (C-1629), hereby incorporated by reference for thatpurpose.

The subject fixed bed process utilizes the above-described materials, ascatalyst or catalyst precursors: the iron-cobalt spinel, or a mixture ofiron-cobalt spinels, of different iron-cobalt atomic ratios, being inadmixture with, isostructural with Fe₃ O₄, and its reduced and carbidedform. The reduced and carbided materials are generally made in situ inthe apparatus, prior to, and during, the carrying out of the hydrocarbonsynthesis process. A full discussion of the spinel and reduced formmaterials, their properties and their preparation are given hereinaboveand need not be reiterated.

Prior to the CO/hydrogen hydrocarbon synthesis fixed bed run, theiron-cobalt spinel is generally conditioned in the apparatus by purgingwith nitrogen to remove reactive gases and then the temperature isincreased to the reaction temperature range. Then the system isgenerally subjected to the above-described hydrogen treatment for asufficient time to insure complete reduction of metal oxides. However,the pressure, space velocity, and temperature during this reduction stepare not critical and can be utilized in the range which is actually usedduring actual hydrocarbon synthesis.

Following the reduction step, the CO/hydrogen feedstream is introducedinto the apparatus catalyst chamber and the pressure, space velocity,temperature, and hydrogen/CO molar ratio are then adjusted as desired,for hydrocarbon synthesis conditions. Optionally, thereduction/carbiding can be carried out concurrently by contact with theCO/H₂ mixture at elevated temperature.

In the process, the hydrogen and CO are used in a molar ratio in thegaseous feedstream of preferably about a 0.5 to 2.5 molar H₂ /CO ratioand more preferably 1:1 to 2:1 molar ratio. Higher and lower molarratios may also be used.

The temperature in the process is generally in the region of about 200°to 350° C. and preferably being 250° to 300° C. Higher temperatures inthe range 300°-350° C. tend to promote higher % CO conversion, lighterproducts, more methane and more CO₂, formed from the water-gas shiftreaction.

The pressure useful in the process is generally conducted in the rangeof about 50 to 1000 psig and preferably about 100 to 300 psig. Higherand lower pressures can also be used.

The space velocity, used in the process is expressed as "standard"hourly space velocity (SHSV) and is generally about 200 to 4000 volumesof gaseous feedstream/per volume of dry catalyst (excluding diluent)/perhour and is preferably in the range of about 400 to 1200 V/V/hr. Higherand lower space velocities can also be used where higher spacevelocities tend to lead to increased olefin contents but decreased % COconversion.

The percent CO conversion obtainable in the subject process whileproviding substantial quantities of C₂ -C₆ olefins, ranges from about 20to 98% and preferably above about 30%. Higher and lower ratiopercentages of CO conversion may also be utilized.

"Total hydrocarbons" produced in the process is related to theselectivity of percent CO conversion to hydrocarbons, being hydrocarbonsfrom C₁ to about C₄₀ and above inclusive, and is generally about 0 to 50percent, and higher, of the total CO converted and the remainder beingsubstantially converted to CO₂.

The percent total C₂ -C₆ hydrocarbons of the total hydrocarbonsproduced, including olefins and paraffins is generally about 20 to 80wt. % and preferably about 50 to 80 wt. %. The percent of C₂ -C₆ olefinsproduced of the C₂ -C₆ total hydrocarbons produced is generally about 50to 90 wt. % and preferably about 70 to 90 wt. % of the C₂ -C₆ totalhydrocarbons. The olefins produced in the process are substantiallyalpha olefins.

The selectivity to methane based on the amount of CO conversion is about2 to 12 weight percent of total hydrocarbons produced. Preferably about10 percent and lower methane is produced in the process.

As discussed above, the percent selectivity to CO₂ formation in theprocess is in the range of about 10 to 50 percent of CO converted, andgenerally about 30 to 50 percent.

The reaction process variables are preferably adjusted to minimize CO₂production, minimize methane production, maximize percent CO conversion,and maximize percent C₂ -C₆ olefin selectivity, while achieving activitymaintenance in the catalyst system.

The catalyst in the process may become contaminated with high molecularweight hydrocarbons on exposure to carbon monoxide hydrogenationreaction conditions. As a result of this catalyst activity may bediminished. In the event that this is observed it may be possible torecover nearly full catalyst activity by exposing the catalyst to asolvent wash and/or hydrogen treatment at elevated temperatures. We havefound that this procedure can in some cases restore the catalyst withits initial performance characteristics.

Generally, this format can be achieved in a preferred mode of operatingthe process where the formula of the catalyst used is Fe₂.85 Co₀.15 O₄/1% K, having about 1 m² /g BET surface area. The pretreatment procedureis conducted at 500° C. in a 9:1 H₂ /N₂ stream @ 680 v/v/hr. under 100psig for 5-7 hours, and the hydrocarbon synthesis run is conducted atthe CO/hydrogen molar ratio is 1:1 to 2:1, the temperature is conductedin the range 230°-270° C., at a pressure of 150-300 psig, and spacevelocity 1000-1800 v/v/hr (SHSV). By carrying out the above process inthe stated variable ranges efficient activity maintenance and productionof C₂ -C₆ olefins can be achieved.

The effluent gases in the process exiting from the reactor may berecycled if desired to the reactor for further CO/hydrocarbon synthesis.

Methods for collecting the products in the process are known in the artand include distillation, fractional distillation, and the like. Methodsfor analyzing the product liquid hydrocarbons and gaseous streams arealso known in the art and generally include gas chromatography, liquidchromatography, high pressure liquid chromatography and the like.

Apparatus useful in the preferred process is any conventional fixed bedtype reactor, being horizontal or vertical, moving bed, fluid bed, andthe like. Other apparatus not specifically described herein will beobvious to one skilled in the art from a reading of this disclosure.

The following examples are illustration of the best mode of carrying outthe claimed invention as contemplated by us and should not be construedas being limitations on the scope and spirit of the instant invention.

EXAMPLE 1

Solid solutions with the generic empirical formula: Fe_(3-y) Co_(y) O₄/1% K (1 gram-atom percent potassium as the carbonate) were prepared bythe following procedure. Mixtures of Fe₂ O₃, Fe metal and Co₃ O₄ in thefollowing molar ratios, (4/3-4y/9) Fe₂ O₃ +1/3 (1-y/3) Fe+y/3 Co₃ O₄,where the value of y independently was: 0, 0.03; 0.150; 0.375; and0.750, corresponding respectively to the following weights in grams ofFe₂ O₃, Fe metal, and Co₃ O₄ ; 21.080, 1.8400, 0.00; 22,750, 1.9891,0.2594; 21.797, 1.9054, 1.2974; 20.0163, 1.7502, 3.2338; 11.381, 0.9590,4.2904. The materials (reagent quality or better from Alfa ChemicalsCo.) were well mixed, placed into a quartz tube, evacuated to 10⁻³ torr,sealed in the tube under vacuum and then heated to 800° C. for 24 hours.The resulting solids were isolated after cooling and breaking the tubeopen, ground to a powder, and resubjected to the same high temperaturesintering procedure, at 800° to 1000° C. for an additional 24 hours.Powder X-ray diffraction analysis was then conducted to ensure that thesintered material was isostructural with pure standard sample of Fe₃ O₄.The catalyst powder was then pelletized and sintered in a sealed tube asdescribed above under vacuum at 1000° C. for several hours. The sinteredpellets were then crushed, seived and the resulting pellets impregnatedwith aqueous potassium carbonate to achieve the desired potassiumloading, being about 1 gram-atom percent potassium, and dried. The BET(nitrogen) surface areas measured were in the range from about 0.25 to0.30 m² /g. The results are listed below in Table I.

                  TABLE 1                                                         ______________________________________                                                      Fe.sub.3-y Co.sub.y O.sub.4 /1% K                               Composition     y       BET (m.sup.2 /g)                                      ______________________________________                                        Control         0.00    0.27                                                  A                0.0275 0.30                                                  B               0.150   0.29                                                  C               0.375   0.25                                                  D               0.750   0.28                                                  ______________________________________                                    

The powder X-ray diffraction spectrum of each of the obtained Fe-Cospinels showed that they were a single phase and isostructural with Fe₃O₄. They differed from one another in slight shifts of the 2 thetareflection values without altering the overall profile.

EXAMPLE 2

Catalyst B, Fe₂.85 Co₀.15 O₄ /1% K, where y=0.15, was prepared by theprocedure described in Example 1. X-ray diffraction analysis showed thismaterial to be isostructural with Fe₃ O₄, although there was a slightchange in the unit cell constant where the unit cell constant is about0.01 to 0.02 Å smaller than that of Fe₃ O₄. The sintered material wasfound to have a low surface area, less than 5 m² /g. This material wascrushed and sieved to 20-80 mesh before use in this example under F-T(Fischer-Tropsch) fixed bed reaction conditions. The reactor was chargedwith 8.8 cc of catalyst with a thermocouple placed at the center of thebed. The catalyst compositions of 20-80 mesh particle size, werepretreated with hydrogen gas in nitrogen (90% hydrogen/nitrogen) at 500°C., 100 sccm (680 v/v/hr.) of hydrogen gas at 100 psig for 5 to 7 hoursin a fixed bed tubular vertical reactor constructed of 316 stainlesssteel, and being 0.51" internal diameter and 15" long. The runs wereconducted using a 1:1 H₂ /CO mixture, at 570 v/v/hr., 300 psig, at theindicated temperatures, which are furnace temperatures in this and theremaining examples unless otherwise indicated as bed temperatures. Inmany of the cases, the bed temperature was 10°-30° C. higher than theindicated furnace temperature, due to primarily to the limited heatremoval capabilities of the reactor system and the highly exothermicnature of the reaction. The overall collected products which werecollected after catalyst pretreatment, and one hour on stream withCO/H₂, were analyzed by gas chromatography.

Representative results obtained with catalyst composition B, Fe₂.85Co₀.15 O₄ /1% K, relative to the control (see Table I) are presentedbelow in Table II.

                  TABLE II                                                        ______________________________________                                        Catalyst     Fe.sub.3 O.sub.4 /1% K.sup.a                                                              Fe.sub.2.85 Co.sub.0.15 O.sub.4 /1%                  ______________________________________                                                                 K.sup.b                                              Temp °C.                                                                            305         270                                                  % CO Conversion                                                                            79          98                                                   % CO to CO.sub.2                                                                           36          42                                                   % CO to HC.sup.c                                                                           43          56                                                   Wt. % Selectivity                                                             CH.sub.4     8.5         9.1                                                  C.sub.2 H.sub.6                                                                            2.1         4.3                                                  C.sub.2 H.sub.4                                                                            6.5         9.8                                                  C.sub.3 H.sub.8                                                                            1.4         1.9                                                  C.sub.3 H.sub.6                                                                            10.6        20.3                                                 C.sub.4 H.sub.10                                                                           1.7         tr.                                                  C.sub.4 H.sub.8                                                                            9.5         9.3                                                  C.sub.5.sup.+                                                                              59.7        45.2                                                 ______________________________________                                         .sup.a Control.                                                               .sup.b Composition B.                                                         .sup.c Hydrocarbons.                                                     

As is seen from the data, Catalyst B, derived from the cobalt-containingspinel, exhibited greater activity at lower temperatures and higher C₂-C₄ olefin selectivity than the all iron control catalyst.

It should be noted that unless stated differently herein, the catalystsused in each of the following examples were in powder form of 20-80mesh, used as is, or diluted with crushed quartz powder, totalling acatalyst volume of about 8-8.8 cc.

Further, the apparatus used was the same as described in this Example 2and the pretreatment procedure was substantially the same as describedin Example 2.

Values for selectivity weight percentages of product hydrocarbons arereported on a CO₂ -free basis unless otherwise stated.

EXAMPLE 3

Four (4) cc. of Catalyst B, described above in Example 2, was mixed with20-80 mesh solid quartz powder (crushed quartz tubes) in 4.0 ccquantity, and the mixture was placed into the reactor described inExample 1, and pretreated by contacting with a 9:1 H₂ /N₂ feedstream at500° C., 750 v/v/hr., 100 psig, for 5.5 hours.

The mixed diluted catalyst was then contacted with 1:1 H₂ /CO at 270°C., 300 psig, at 2000 v/v/hr. for 12 hours on stream. The productdistribution was analyzed by gas chromatography, and the results aregiven below in Table III.

                  TABLE III                                                       ______________________________________                                        Catalyst     1:1 Catalyst B/quartz powder                                     ______________________________________                                        % Conversion 62                                                               % CO to CO.sub.2                                                                           24                                                               % CO to H.C. 38                                                               Wt. % Selectivity                                                             CH.sub.4     9.2                                                              C.sub.2 °-C.sub.5 °                                                          7.9                                                              C.sub.2.sup.= -C.sub.5.sup.=                                                               48.2                                                             C.sub.6.sup.+                                                                              34.7                                                             ______________________________________                                    

As is seen from the data, the catalyst derived fom the iron-cobaltspinel provides good activity and high C₂ -C₅ olefin selectivity withhigh H₂ /CO feed rates.

EXAMPLE 4

Catalyst B, in a 1:1 admixture with crushed quartz, as described inExample 3, was run under a different set of F-T synthesis conditions asdescribed below.

Following substantially the same pretreatment, described in Example 3,about 8 cc of the catalyst in the same described apparatus as above wascontacted with 1:1 H₂ /CO, at a bed temperature of 250° to 270° C., astandard hourly space velocity (SHSV) of 1000 v/v/hr. at 300 psig, for12 hours. The products were collected and the product distribution datawere analyzed by gas chromatography. Results are given below in TableIV.

                  TABLE IV                                                        ______________________________________                                        % CO conversion     98                                                        % CO to CO.sub.2    43                                                        % CO to HC          55                                                        Wt. % Selectivity CH.sub.4                                                                        7.2                                                       C.sub.2.sup.= /C.sub.2 °                                                                   2.6                                                       C.sub.2 /C.sub.1    2.1                                                       % C.sub.2 -C.sub.6  50.8                                                      % Olefins (of C.sub.2 -C.sub.6 total)                                                             86                                                        C.sub.7.sup.+       42                                                        ______________________________________                                    

As is seen from the data, the Fe-Co Catalyst B generates a C₂ -C₆fraction which is olefin rich even at high conversion conditions.

EXAMPLE 5

Catalyst B and the control, prepared by the procedure described inExample 1, were pretreated by the procedure described in Example 3 inthe apparatus described in Example 2.

Each catalyst in 8 cc volume, after pretreatment, was contacted with 1:1H₂ /CO at 300 psig pressure, 1000 v/v/hr. (SHSV) for 12 hour run timesat the temperatures listed below in Table VI, in same apparatusdescribed in Example 2. Product samples were collected and analyzedafter 12 hours onstream with CO/H₂.

                  TABLE VI                                                        ______________________________________                                                     Catalyst B                                                                             Control   Control                                       ______________________________________                                        % CO Conversion                                                                              98         67        87                                        % CO to CO.sub.2                                                                             40         31        37                                        % CO to HC     58         36        50                                        Temp. °C.                                                                             270        305       340                                       C.sub.2 :C.sub.1                                                                             2.2        1.2       0.7                                       % C.sub.2 -C.sub.6                                                                           62         41        53                                        % Olefin (of C.sub.2 -C.sub.6 total)                                                         89         88        70                                        Weight % Selectivity                                                          C.sub.1        7.4        5.8       19.0                                      C.sub.2 °                                                                             4.4        1.3       7.8                                       C.sub.2.sup.=  11.6       5.4       5.7                                       C.sub.3 °                                                                             1.5        1.0       2.6                                       C.sub.3.sup.=  20.0       9.4       15.9                                      C.sub.4 °                                                                             tr.        1.4       2.0                                       C.sub.4.sup.=  11.3       8.8       8.6                                       C.sub.5 °                                                                             0.3        1.0       1.1                                       C.sub.5.sup.=  7.4        7.0       4.0                                       C.sub.6 °                                                                             0.8        0.3       2.6                                       C.sub.6.sup.=  4.6        5.0       3.0                                       C.sub.7.sup.+  30.7       53.6      27.7                                      ______________________________________                                    

As is seen from the data, the catalyst derived from the cobaltcontaining spinel provided greater activity, i.e. 98% CO conversion,than the all-iron oxide control catalysts even though they were operatedat 35° C. and 70° C. higher temperatures. The Fe-Co catalyst generatedmore C₂ -C₆ olefins than either of the control catalysts andsubstantially less methane than the control catalyst at high conversion(about 87%) conditions.

EXAMPLE 7 Catalyst Preparation

Following the general procedure described in Example 1 the followingcatalysts were prepared having the empirical formula: Fe_(3-y) Co_(y) O₄/1%K: where y=0.03, 0.15, 0.375 and 0.75, respectively. The surfaceareas of the obtained materials were in the range of 0.1 to 0.5 m² g.

The above-prepared catalysts were pretreated by the procedure describedin Example 2 and in the apparatus described in Example 4, and subjectedto hydrocarbon synthesis under the following reaction conditions:

Temperature=295±10° C.

Pressure=300 psig

Space Velocity=1000 v/v/hr.

H₂ /CO ratio=1:1

Run Time=12 hours

Catalyst=8 cc volume, 20-80 mesh size

Analysis of products were performed after 12 hours of run time. Resultsare shown in Table VII below.

                  TABLE VII                                                       ______________________________________                                        Performance of Fe.sub.3-y Co.sub.y O.sub.4 /1% K                              ______________________________________                                        y =            0.03   0.15     0.375 0.80                                     % CO Conversion                                                                              97     98       97    98                                       To CO.sub.2    27     40       41    42                                       To HC's        70     58       56    56                                       Wt. % Selectivity                                                             CH.sub.4       8.3    7.4      18.0  13.2                                     C.sub.2.sup.= -C.sub.6.sup.=                                                                 46.5   53.1     41.4  53.0                                     C.sub.2 °-C.sub.6 °                                                            6.9    7.2      13.3  10.6                                     C.sub.7.sup.+  38.3   32.3     27.3  23.2                                     ______________________________________                                    

The results show the importance of maintaining the Fe:Co atomic ratiowithin the preferred range i.e. y=0.03 to y=0.40 at the specificconditions in this Example, excessive levels of CH₄ are generated athigh cobalt levels, i.e., y=0.375 where Fe:Co=7:1.

EXAMPLE 8

This example shows the performance of Catalyst C, Fe₂.625 Co₀.375 O₄ inhydrocarbon synthesis at different temperatures.

The catalyst was pretreated according to the procedure described inExample 2 and in the same described apparatus. The hydrocarbon synthesisruns were conducted at the indicated temperatures using 8 cc. volume ofcatalyst being undiluted with quartz and 20-80 mesh particle size at 1:1H₂ /CO, 1000 v/v/hr. (SHSV), 300 psig for 1-12 hours onstream.

                  TABLE VII                                                       ______________________________________                                        Performance of Fe.sub.2.625 Co.sub..375 O.sub.4 /1% K                         ______________________________________                                        Furnace     225     240    260   270  280   290                               Temp °C.                                                               Bed Temp °C.                                                                       230     248    304   325  331   340                               % CO Conversion                                                                            30      31     97    98   98    98                               To CO.sub.2  4       7      40    33   41    41                               To HC's      26      24     57    55   57    57                               Wt. % Selectivity - CO.sub.2 -free basis                                      CH.sub.4     8.1     8.2   19.1  16.7 18.3  19.1                              C.sub.2.sup.= -C.sub.5.sup.=                                                              42.3    55.3   37.1  31.9 37.8  24.8                              C.sub.2 °-C.sub.5 °                                                         14.4    22.0   17.7  10.6 13.2  14.8                              C.sub.6.sup.+                                                                             35.2    34.5   26.1  40.8 30.7  41.3                              ______________________________________                                    

As seen from the data, the change in CH₄ selectivity as a function oftemperature-conversion indicates that catalysts which contain relativelyhigh levels of cobalt, i.e. an iron/cobalt atomic ratio of 7.0, whileuseful should be operated at lower temperature-conversion conditions toachieve low CH₄ productivity. As further seen in the data, good C₂ -C₆olefin selectivity is achieved over the entire operating range. Thesystem provided optimal performance in runs where the bed temperaturewas lower than 304° C.

EXAMPLE 9

This example shows the improved performance of Catalyst C, Fe₂.625Co₀.375 O₄, at low (150 psig) pressure relative to (300 psig) highpressure conditions. The catalyst was prepared by the procedure outlinedin Example 1, and subjected to the pretreatment and operating proceduressubstantially as described in Examples 2 and 4, respectively.

The results in Table VIII below show that even at relatively high cobaltlevels, i.e. Fe:Co of 7.0, good olefin selectivity and high conversioncan be achieved at lower pressures, i.e. 150 psig.

                  TABLE VIII                                                      ______________________________________                                        Performance of Fe.sub.2.625 Co.sub..375 O.sub.4 /1% K                         at 150 and 300 psig                                                           ______________________________________                                        Pressure (psig)    150    300                                                 % CO Conversion     92     97                                                 % To CO.sub.2       38     41                                                 % To HC             54     56                                                 Wt. % Selectivity (CO.sub.2 -free basis)                                      CH.sub.4            7.2   17.9                                                C.sub.2.sup.= -C.sub.5.sup.=                                                                     53.4   38.1                                                C.sub.2 °-C.sub.5 °                                                                 4.5   12.7                                                C.sub.6.sup.+      34.9   31.3                                                ______________________________________                                    

EXAMPLE 10

This example shows the effect of H₂ treatment at 350° C. to reduce CH₄selectivity of an "aged catalyst", in this Catalyst B, which had beenonstream for 72 hours. It is believed that the treatment with H₂ at 350°C. for 5 hrs. at 100 psig, 750 SHSV, removes a carbonaceous surfacelayer which develops on the catalyst during extended operating periods.The procedures described in Examples 1, 3 and 3 were used torespectively prepare, pretreat, and operate this catalyst under thehydrocarbon synthesis conditions of 270° C., 0.66:1 H₂ /CO, 2000 v/v/hr.(SHSV), 300 psig, 50% catalyst dilution with quartz powder in 8 cc totalvolume, catalyst particle size of 20-80 mesh.

                  TABLE IX                                                        ______________________________________                                        H.sub.2 Treatment Improves Time Dependent                                     Performance of Fe.sub.2.85 Co.sub..15 O.sub.4 /1% K                           ______________________________________                                        Hours on stream    .sup. 72.sup.a                                                                       .sup. 96.sup.b                                      % CO Conversion    48     62                                                  % CO to CO.sub.2   23     28                                                  % CO to HC         25     34                                                  Wt. % Selectivity (CO.sub.2 -free basis)                                      CH.sub.4           12.0    7.9                                                C.sub.2.sup.= -C.sub.5.sup.=                                                                     43.3   46.3                                                C.sub.2 °-C.sub.5 °                                                                 7.1    6.6                                                C.sub.6.sup.+      37.6   40.1                                                ______________________________________                                         .sup.a Prior to hydrogen rejuvenation.                                        .sup.b After 72 hours onstream, H.sub.2 treatment described above, then       additional 24 hours onstream with CO/H.sub.2.                            

EXAMPLE 11

This example demonstrates the performance of Catalyst B, Fe₂.85 Co₀.15O₄, at various temperatures under hydrocarbon synthesis conditions. Thecatalyst was 50% diluted with quartz powder as described in the previousExample. The respective procedures outlined in Examples 1 and 3 wereused to prepare, pretreat and operate this catalyst under thehydrocarbon synthesis conditions listed below in Table X.

                  TABLE X                                                         ______________________________________                                        Fe.sub.2.85 Co.sub..15 O.sub.4 /1% K Performance                              Run       1           2           3                                           ______________________________________                                        Temp °C.                                                                         230             250         270                                     Pressure  300             300         300                                     (psig)                                                                        H.sub.2 /CO                                                                             1.0             1.0         1.0                                     SHSV      1800            1800        1800                                    % CO Conv.                                                                              36.4            97.5        98.4                                    HR on Stream                                                                            2               4           6                                       % CO to CO.sub.2                                                                        14              44.0        43.0                                    % CO to HC                                                                              22.4            53.5        55.4                                    Wt. % Selectivity (CO.sub.2 -free basis)                                      CO.sub.2  37.9            45.0        43.8                                    CH.sub.4  1.3    (2.1)    2.6  (4.7)  3.2  (5.7)                              C.sub.2.sup.=                                                                           2.0    (3.22)   3.0  (5.5)  3.4  (6.0)                              C.sub.2 °                                                                        0.4    (0.6)    0.8  (1.5)  0.8  (1.4)                              C.sub.3.sup.=                                                                           4.1    (6.6)    5.4  (9.8)  6.3  (11.2)                             C.sub.3 °                                                                        0.8    (1.3)    0.6  (1.1)  0.6  (1.1)                              C.sub.4.sup.=                                                                           1.7    (2.7)    3.4  (6.2)  4.0  (7.1)                              C.sub.4 °                                                                        0.1    (0.2)    0.5  (0.9)  0.5  (0.9)                              C.sub.5.sup.=                                                                           1.4    (2.3)    2.6  (4.7)  3.5  (6.2)                              C.sub.5 °                                                                        0.3    (0.5)    0.5  (0.9)  0.9  (1.6)                              C.sub.6.sup.=                                                                           1.2    (1.9)    1.9  (3.5)  2.2  (3.9)                              C.sub.6 °                                                                        0.4    (0.6)    0.3  (0.5)  0.3  (0.5)                              C.sub.7.sup.+                                                                           48.4   (78.0)   33.4 (60.7) 30.5 (54.4)                             ______________________________________                                    

EXAMPLE 12

This example demonstrates the performance of Catalyst B, Fe₂.85 Co₀.15O₄ at various temperatures in the form of undiluted catalyst. Thecatalyst was prepared by the procedure described in Example 1 andpretreated and operated as respectively described in Examples 2 and 4.The process conditions for each run are listed below in Table XI. Incontrast to Run 4 shown below, bed dilution as employed in Example 10allows the system to operate under more isothermal conditions therebyminimizing the extent of carbon and carbonaceous deposit formation.

                  TABLE XI                                                        ______________________________________                                        Fe.sub.2.85 Co.sub..15 O.sub.4 /1% K Performance                              Undiluted Bed                                                                 Run          1       2         3     4                                        ______________________________________                                        SHSV:        1000    1000      570   570                                      Temp.        235     270       235   270                                      H.sub.2 :CO  1.0     1.0       1.0   1.0                                      Press        300     300       300   300                                      Time on       8       10        16    18                                      stream hr.                                                                    % CO Conv.   29.4    98.0      49.1  98.0*                                    % CO to CO.sub.2                                                                           8.0     42.0      22.0  40.0                                     % CO to HC   21.4    56.0      27.1  58.0                                     Wt. % Select. (CO.sub.2 -free basis)                                          CO.sub.2     26.2    42.5      43.5  40.2                                     CH.sub.4     1.9     3.0       1.7   3.7                                                   (2.6)   (5.2)     (2.0) (6.2)                                    C.sub.2.sup.=                                                                              4.3     4.5       2.5   4.0                                                   (5.8)   (7.8)     (4.4) (6.7)                                    C.sub.2 °                                                                           1.4     1.7       0.9   1.7                                                   (1.9)   (2.9)     (1.6) (2.8)                                    C.sub.3.sup.=                                                                              6.4     7.8       6.6   8.3                                                   (8.7)   (13.4)    (11.6)                                                                              (13.8)                                   C.sub.3 °                                                                           0.6     0.6       0.7   0.8                                                   (0.8)   (1.0)     (1.2) (1.3)                                    C.sub.4.sup.=                                                                              1.4     4.4       2.5   3.8                                                   (1.9)   (7.6)     (4.4) (6.3)                                    C.sub.4 °                                                                           tr.     0.2       0.4   3.5                                                   (tr.)   (0.3)     (0.7) (0.8)                                    C.sub. 5.sup.=                                                                             0.9     2.8       1.7   2.7                                                   (1.2)   (4.8)     (2.9) (4.5)                                    C.sub.5 °                                                                           tr.     0.1       0.4    0.35                                                 (tr.)   (0.2)     (0.7) (0.8)                                    C.sub.6.sup.+                                                                              56.9    32.4      39.1  34.2                                                  (76.8)  (55.9)    (68.6)                                                                              (57.0)                                   ______________________________________                                         *Note:                                                                        Bed plugging with wax and carbonaceous deposits limited continuous            operating periods to ≦ 40-50 hrs.                                 

What is claimed is:
 1. A fixed bed process for synthesizing ahydrocarbon mixture containing C₂ -C₆ olefins comprising the step ofcontacting a fixed bed of a catalyst composition comprised of anunsupported Group IA or IIA metal salt promoted iron-cobalt spinel: saidspinel exhibiting a single spinel phase, being isostructural with Fe₃ O₄as determined by X-ray diffractometry and possessing a BET nitrogensurface area of up to 5 m² /g, and an iron-cobalt atomic ratio of 7:1 orabove; with a mixture of CO/hydrogen under process conditions ofpressure, space velocity (SHSV) and elevated temperature for a timesufficient to produce said C₂ -C₆ olefins.
 2. The process of claim 1wherein said hydrogen and CO are present in a molar ratio of about 0.5to 2.5.
 3. The process of claim 1 wherein said temperature is in a rangeof about 200° to 350° C.
 4. The process of claim 1 wherein said pressureis in a range of about 50 to 1000 psig.
 5. The process of claim 1wherein said space velocity is in the range of about 200 to 4000 V/V/hr.6. The process of claim 1 wherein said spinel is of the formula: Fe_(x)Co_(y) O₄, wherein x and y are integer or decimal values other thanzero, the sum of x+y is 3 and the ratio of x/y is 7:1-35:1.
 7. Theprocess of claim 6 wherein the ratio x/y is 19-20:1.
 8. The process ofclaim 6 wherein said spinel is of the formula: Fe₂.85 Co₀.15 O₄, Fe₂.625Co₀.375 O₄, Fe₂.97 Co₀.03 O₄.
 9. The process of claim 1 wherein saidcatalyst is further in admixture with a solid diluent which aids in heattransfer and removal from the catalyst bed.
 10. The process of claim 1wherein said Group IA or IIA metal promoter is present in about 0.1 to10 gram-atom % as the metal ion of the total gram-atoms metal content.11. The process of claim 10 wherein said promoter is selected frombicarbonates, carbonates, organic acid salts, inorganic acid salts,nitrates, sulfates, halides and hydroxides of Group IA and IIA metals.12. The process of claim 11 wherein said promoter is potassiumcarbonate.
 13. The process of claim 1 wherein said product hydrocarbonmixture contains about 20 wt. % and above C₂ -C₆ hydrocarbons of thetotal weight of hydrocarbons produced.
 14. The process of claim 13wherein said C₂ -C₆ hydrocarbons contain C₂ -C₆ olefins as about 50 wt.% and above of said total C₂ -C₆ hydrocarbons.
 15. A fixed bed processfor synthesizing a hydrocarbon mixture containing C₂ -C₆ olefinscomprising the step of contacting a fixed bed of a catalyst compositioncomprised of an unsupported iron-cobalt spinel of the formula: Fe₂.85CO₀.15 O₄, containing about 1 gram-atom percent potassium as thecarbonate, wherein said spinel exhibits a single spinel phase beingisostructural with Fe₃ O₄ as determined by X-ray diffractometry andinitially possessing a surface area of about 1 m² /g, with a 2:1 to 1:1H₂ /CO mixture, at about 150-300 psig, 1000-1800 v/v/hr (SHSV) and about230°-270° C. for a time sufficient to produce said C₂ -C₆ olefins.